This invention relates to distillation residues resulting from the preparation of toluene diisocyanate. More particularly, the invention relates to the preparation of viscosity-stabilized toluene diisocyanate distillation residues for use in the preparation of polyurethane and related products.
Toluene diisocyanates (hereinafter TDI) are commonly prepared by phosgenation of toluene diamines. Typical processes for the phosgenation of amines can be found in, for example, U.S. Pat. Nos. 2,680,127; 2,822,373; and 3,781,320. In the phosgenation of toluene diamines to prepare toluene diisocyanates the product diisocyanate is generally distilled from the reaction mixture in which it is prepared. At the conclusion of the distillation the reaction mixture normally contains a quantity of high boiling residue. Such residue generally comprises polymeric materials such as alpha, omega-isocyanatobiurets, polycarbodiimides, diisocyanato-carbodiimides, polyuretidinediones, isocyanurates and various other isocyanate adducts, such as hydrolyzable chlorides. This residue is typically discarded.
The disposal of this residue is a serious problem to the TDI industry. It is costly and poses safety problems. The handling of this material is a problem because special equipment is needed to move and store it prior to disposal.
Thus, it is desirable in the art to develop means of using these TDI distillation residues. A number of treatment methods have been developed, which include processes designed to convert the residues to entirely new materials, processes to use them as fillers in various plastics materials, and processes designed to "purify" them. For example, U.S. Pat. No. 4,506,040 discloses a process wherein TDI residue is reacted with an active hydrogen containing compound to form a polymer. The polymer is then dispersed in an organic solvent. A high molecular weight polyol is added and the solvent is removed, thereby producing a dispersion of the polymer in the high molecular weight polyol. The resulting dispersion is used to prepare polyurethane products, including foams and elastomers.
U.S. Pat. No. 4,480,081 discloses a process wherein a TDI distillation residue is reacted with a monohydroxylic compound, and the modified product is then reacted with a polyol. The final product is an isocyanate reactive compound for use in polyurethane formulations. U.S. Pat. No. 4,311,800 discloses a process wherein highly cross-linked insoluble distillation residues are reacted with water and then alcohol to form soluble compounds ontaining hydroxyalkyl and urethane groups. They may also be recyclized by simple hydrolysis into toluene diamines (TDA), which is the starting material from which TDI is derived. U.S. Pat. No. 4,489,177 discloses reaction of the TDI residue with a polyol to obtain a product having an OH-number of from 200 to 700. U.S. Pat. No. 3,455,836 discloses addition of 4,4'-methylene bis(phenylisocyanate) to liquid TDI distillation residue. The product can be reacted with active hydrogen containing materials.
German Patent 2,846,814 describes a process by which TDI bottoms are pulverized, treated with NCO-reactive compounds, and then dispersed in aliphatic polyols. The dispersions are used to manufacture urethane polymers. Another German Patent, 2,942,678, discloses mixing ground residues with certain monohydric alcohols, diols, or triols to produce a mixture containing at least 2 percent hydroxyl groups; heating the mixture at greater than 130.degree. C. until homogeneous; and then cooling and grinding the product to use as a starting material in plastics. U.S. Pat. No. 4,595,709 shows preparation of polyaddition products containing urethane groups by reacting the TDI distillation residue with a compound containing hydroxyl groups such that the NCO/OH equivalent ratio is less than 1.5:1. The product can be used to prepare flame-resistant polyurethanes. U.S. Pat. No. 4,293,456 discloses use of the TDI residues directly as a reactive filler in polyurethanes to improve mechanical properties. U.S. Pat. No. 4,251,401 discloses producing suspension of the residues in polyhydroxyl compounds to use as the polyol component in forming polyurethanes. German Patent 2,846,815 discloses milling of the TDI residues, followed by treatment with isocyanate-reactive compounds to prepare reactive materials which are useful as fillers or dispersants.
None of the above methods allows use of the TDI distillation residues directly as the isocyanate component of a polyurethane formulation. In general TDI distillation residues are too acidic to be useful as such. Such acidity is often attributalbe, at least in part, to the presence of high levels of hydrolyzable chlorides, which respond as acids in standard analytical tests. The term "hydrolyzable chloride" as used herein refers to labile chlorine atoms which are ionically or covalently bonded within a compound, but are more reactive than, for example, the chlorine atom present in chlorobenzene. Hydrolyzable chloride concentration may also be referred to as chloride equivalent or chloride level. These hydrolyzable chlorides inhibit the reaction of the residues with active hydrogen compounds and therefore, when present in sufficient amount, may ultimately preclude use of the residues in preparing satisfactory polyurethane products.
Means to remove hydrolyzable chlorides include, for example, the process described in U.S. Pat. No. 4,904,704. That patent discloses a method in which the residues are reacted with at least about 0.5 equivalents, based on hydrolyzable chloride concentration in the residues, of an epoxy compound at an elevated temperature. The product has a hydrolyzable chloride concentration of less than about 800 parts per million. The resulting treated residue can be reacted, either alone or blended with other isocyanates, with an active hydrogen compound to form a polyurethane.
However, a problem encountered when TDI distillation residues, including those treated to reduce acidity and/or hydrolyzable chlorides, are used alone or blended with other polyisocyanates is that the products tend to be unstable over time with respect to viscosity. This presents problems in processing, and also a major obstacle to large-scale standardized production operations. Because of these problems those skilled in the art have avoided using TDI residues to prepare such polyurethanes.
It would be desirable in the art to find a process to prepare a toluene diisocyanate distillation residue that is relatively stable in viscosity and which also has a desirable reactivity when used to prepare a polyurethane product. By "viscosity stable" as used herein is meant that the residue increases in viscosity, if at all, more slowly than residues which have not received additional processing after their formation upon distillation from crude TDI.